The purpose of this study was to develop a mastery-based adapted aquatics program following an educational research and development model, and to ascertain the potential effectiveness of that program. Factors that were investigated in this study included student performance time, teacher feedback rate, and teacher perceptions of instructional effectiveness. The program was staffed by volunteers. The participants were mentally retarded individuals, and represented a moderate range of ability levels. Mastery sequences were developed for a wide majority of swim skills. Each sequence contained the skill to be learned, the tasks which combine to form the skill, and a criterion to be reached for mastery of each task. Instructors were provided with preliminary and on-going training in the utilization of the mastery sequence cards and the provision of feedback. Data were collected before and during inception of the mastery based program using a naturalistic observation system, and teacher surveys. The results showed a significant increase in the rate of teacher feedback provided, a significant decrease in the amount of off-task time, and an overall positive instructor attitude toward the mastery sequence cards. There was not a significant difference in the amount of performance time although a probable cause was determined to be a lack of sequence cards available. Possible modifications were discussed, as well as recommendations for future research. / M.S.